1. Field of the Invention
The present invention relates to a radio communication apparatus and more preferably to a radio communication apparatus used in a mobile communication system and a radio LAN system employing the UMTS (WCDMA) communication system.
2. Description of the Related Art
In recent years, data transmission utilizing a shared channel, which is capable of switching a transmission destination of data in accordance with the situation, is most frequently accomplished in place of the communication which is conducted by assigning a fixed time slot.
In the data transmission system utilizing the shared channel, a communication apparatus on the receiving side (referred to as a receiving station) cannot foresee when data is transmitted from a communication apparatus on the transmitting side (referred to as a transmitting station). Therefore, the receiving station checks whether the data transmitted is destined to its own station or not by receiving every time data is transmitted from the transmitting station. When the data received is transmitted to its own station, the receiving station extracts and processes (decodes) the received data and thereafter outputs the data. However, when the received data is not transmitted to its own station, the receiving station is not required to decode and output the data.
Moreover, in the data transmission system utilizing the shared channel, it is sometimes required to utilize information from the receiving station at the time of transmitting the data.
For example, the information from the receiving station is used, when adaptive modulation and coding (AMC) is conducted for transmission of data.
The adaptive modulation and coding is necessary for adaptively changing the transmission method for the data transmission.
For example, a receiving station receives the signal transmitted from a transmitting station, measures the radio environment between the transmitting station and the receiving station (radio environment of the downlink) using such receiving signal, and then transmits the result of measurement to the transmitting station, while the transmitting station changes adaptively the transmission method on the basis of the result of measurement. Examples of changes in the transmission method include conversion of the QPSK modulation system into the 16QAM modulation system, a change in the number of the spreading codes used by the transmission, and a change in the data size (packet length) to be transmitted may be thought.
Here, the HSDPA (High Speed Downlink Packet Access) will be explained as an example of a system for data transmission utilizing a shared channel. The HSDPA is a system which is adopted into the UMTS (WCDMA) communication system to enable a high speed packet transmission system using the shared channel of the downlink. Moreover, in the radio LAN communication system, the high speed packet transmission system using the shared channel has also been proposed.
The HSDPA is employed not only on data transmission utilizing the shared channel, explained previously, but also to adaptive modulation and coding (AMC).
In addition, the HSDPA also employs the H-ARQ (Hybrid Automatic Repeat request) system. In this H-ARQ system, a re-transmission request is issued to a base station when a mobile station has detected an error in the received data from a base station. The base station having received this re-transmission request performs re-transmission of data. Accordingly, the mobile station is capable of effectively using the data already received and the received data of the re-transmission. As explained above, generation of useless transmission is eliminated by effectively utilizing the data already received.
Next, the principal radio channel used in the HSDPA will be explained.
The channels of the HSDPA include the HS-SCCH (High Speed-shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
The channels HS-SCCH and HS-PDSCH are shared channels for downlink (namely, the direction to the mobile station from the base station), while the channel HS-SCCH is a control channel for transmitting a variety of parameters for the data to be transmitted by the HS-PDSCH channel. Various parameters include, for example, the modulation scheme information indicating the modulation scheme to be employed, the number of spreading codes to be assigned, and information such as the pattern of the rate matching process to be accomplished before transmission.
Meanwhile, the HS-DPCCH is an individual control channel (Dedicated Control Channel) for the uplink (namely, the direction to the base station from the mobile station). For example, this channel is used by the mobile station to transmit, to the base station, the result of acknowledgment or non-acknowledgment of reception of data received from the base station via the HS-PDSCH channel respectively as the ACK signal and NACK signal (response signal). If a mobile station fails in reception of data (a CRC error is generated in the received data), a base station accomplishes re-transmission control because the NACK signal is transmitted from a mobile station as the re-transmission request. Moreover, when neither the ACK signal nor the NACK signal are received (in the case of the DTX state), the radio base station also conducts re-transmission control. Accordingly, occurrence of the DTX state in which the mobile station does not transmit the ACK and NACK signals may also be considered as one of the re-transmission requests.
Moreover, the HS-DPCCH channel is also used to transmit receiving quality information (for example, SIR) of the received signal from the base station measured by the mobile station to the base station as the CQI information (Channel Quality Indicator). The base station varies a transmission method of the downlink with the CQI information received. Namely, when the CQI information indicates a better radio environment of the downlink, the transmission method is switched to a modulation scheme enabling higher speed transmission of data. When, on the contrary, the CQI information does not indicate a good radio environment of the downlink, the transmission method is switched to a modulation scheme enabling low speed transmission of data (namely, adaptive modulation control (adaptive modulation and coding).
[Channel Format]
Next, a channel format of the HSDPA channel will be explained.
FIG. 1 is a diagram illustrating the channel format of the HSDPA channel. Since the W-CDMA employs the code division multiplex system, each channel is divided by codes.
The channels not yet explained will be explained first. The CPICH (Common Pilot Channel) is the common channel for the downlink which is used to transmit data to all mobile stations in the radio zone (cell).
The CPICH channel is used in the mobile station for channel estimation and cell search and used as the timing standard for the other downlink physical channel in the same cell also. Namely, this channel is used to transmit the so-called pilot signal.
Next, timing relationships of the channels will then be explained with reference to FIG. 1.
As illustrated in FIG. 1, one frame (10 ms) is formed with 15 slots (=3×5, each slot corresponds to a 2560-chip length) in each channel. As is already explained above, since the CPICH is used as the other channel standard, the first frame of the frames in the P-CCPCH and HS-SCCH channels is matched with the first frame of the frames in the CPICH channel. Here, the first frame of the frames in the HS-PDSCH is delayed by two slots from the HS-SCCH or the like for enabling demodulation of the HS-PDSCH with the demodulation scheme corresponding to the modulation scheme received after the mobile station has received the modulation scheme information via the HS-SCCH. Moreover, the HS-SCCH and HS-PDSCH form one sub-frame with three slots. A mobile station receives every sub-frame in the HS-SCCH to check whether data is transmitted or not to its own station. When the data is transmitted to its own station, the mobile station receives (decodes) the HS-PDSCH. When the data is not transmitted to its own station, the mobile station does not receive (decodes) the HS-PDSCH.
The HS-DPCCH is the channel of the uplink including the slot (one slot length) which is used for transmitting the ACK/NACK signal as the response signal for verifying reception to the base station from the mobile station after about 7.5 slots from reception of the HS-PDSCH.
Moreover, the HS-DPCCH is also used for periodic feedback transmission of the CQI information for adaptive modulation and coding to the base station. Here, the CQI information to be transmitted is calculated, for example, on the basis of the receiving environment (for example, the result of SIR measurement of the CPICH) measured in the period up to before one slot from before four slots of the transmission of CQI information.
Details about the HSDPA are disclosed, for example, in the non-patent document “3G TS 25. 212 (3rd Generation Partnership Project Technical Specification Group Radio Access Network Multiplexing and channel coding (FDD)) V6.2.0 (June, 2004)”.
According to the background art explained above, application efficiency of channels can be improved by utilizing the shared channels, but since a receiving station receives the HS-SCCH with every sub-frame, if the data received not transmitted to its own station, it is a problem to be solved, from the viewpoint of the receiving station itself, that power is consumed for a useless receiving process.
Such problem will be explained briefly with reference to FIG. 2.
FIG. 2 illustrates the transmitting and receiving sequence of the HSDPA channel.
In FIG. 2, only the signals to be transmitted and received between the radio base station (BTS) and one mobile station (MS1) are shown. However, the signals are, of course, likely to be transmitted to other mobile stations in the blank areas of the HS-SCCH and HS-PDSCH channels.
As illustrated in the figure, the MS1 receives the CPICH, which is always transmitted from the BTS, and periodically transmits the result of the measurement to the BTS as the CQI information.
For example, when the MS1 transmits the CQI at time A in the figure and the BTS selects this MS1 as the next transmitting destination of the data, the HS-SCCH (HS-PDSCH (DSCH)) to be transmitted to the MS1 at time B(C) in the figure becomes the data generated on the basis of this CQI information.
Namely, the BTS selects the transmission method corresponding to the CQI, transmits the modulation scheme and the spreading code information corresponding to such transmission method, and effectively accomplishes adaptive modulation and coding by transmitting the HS-PDSCH using such modulation scheme and spreading code.
However, there is no data, in some cases, to be transmitted to the MS1 (transmission of data to the other MS has the priority), and if this period is continued, the period in which the packet is not transmitted is likely to be generated in the MS1.
During this period, the MS1 is forced to accomplish the useless receiving processes (reception of the HS-SCCH or the like) and transmitting process (transmission of the CQI or the like) and uselessly consumes electrical power.